Many materials are stored out-of-doors in bulk form; i.e., in large piles. These include sawdust, coal, grain, mine tailings, various ores and minerals, sand and gravel, wood chips, bark, and a host of other materials having fractions that, because of their size-weight relationship, easily become airborne.
Pollution of the ambient environment is a major problem in the out-of-doors storage of such particulate materials. Even relatively mild winds can pick up and entrain the material as it is being dumped to add onto or to form a pile and can dislodge and scatter material from the pile.
Scattering of particulate solids to any considerable extent from materials piled out-of-doors is at present time considered an environmental threat, and Federal as well as local jurisdictions have enacted legislation limiting the extent to which the entrainment in air of even benign particulates is permitted.
Numerous solutions to the problem have been employed and proposed. Those with which I am particularly familiar deal with the handling and storage of sawdust, and this disclosure will accordingly hereinafter be developed primarily with reference to that material. It is to be understood, however, that this is being done primarily for the sake of conciseness and convenience and is not intended to limit the scope of my invention as defined in the appended claims.
Schemes for preventing the scattering of sawdust and other particulate materials which include significant amounts of fines include enclosure of the pile. This is economically impractical. A typical sawdust pile as located at a pulp mill, for example, will be fifty feet high, seventy-five feet wide at the base, and three-four hundred feet long. An enclosure capable of housing such a pile would cost two-four million dollars at the present time.
Furthermore, in the instances in which this approach has been adopted, it has been found that the particulate material hangs up on the walls and other structural components of traditionally designed enclosures, and bridges between such components, making it difficult, and therefore costly, to remove the material as it is needed. Expensive redesign and re-engineering of conventional structures would be required to solve this problem.
Yet another disadvantage of enclosing a pile of particulate material is that the circulation of air around the pile of particulate material and the radiation of heat from the pile are severely restricted, and cooling of the pile via these two important heat transfer mechanisms is consequently reduced to a significant extent. Therefore, spontaneous combustion becomes a serious problem. This is handled by spraying the pile of particulate matter with water. Spraying is expensive in itself and because the material removed from the pile may later have to be dried as when it is to be used as a fuel, for example.
Another, equally impractical, heretofore proposed solution to the scattering problem has been to at least partially bury the pile of particulate material; i.e., to put the pile in a pit. This, too, is costly. Furthermore, in at least one instance of which I am aware, the solution was not effective; air pollution still exceeded statutory limits, primarily because burial does not solve the important problem of preventing entrainment of fines while the particulate material is being dumped.
In addition, ground water, accumulation of rain, and spontaneous combustion due to the restriction of air circulation and reduced dissipation of heat by radiation make the burial solution impractical.
Fencing akin to that employed to keep snow from drifting has also been proposed. Again, the solution is impractical. As far as I am aware, no one has as yet discovered how to build a fifty foot high snow fence at an acceptable cost, primarily because of the huge wind loads such a fence would have to be designed to withstand. To enclose a pile of particulate material of the size discussed above, for example, would exceed one million dollars.
Covering the sides of a pile of particulate material with netting has also been proposed. Again, the solution is impractical. Large particles hang up in, and plug, the interstices of the netting. The result is that the netting in effect becomes a huge sail. Materials and holddowns capable of handling the resulting wind loads would be prohibitively expensive.
Baffles equaling the height of the pile of particulate material and located on its windward side have also been proposed. This solution is not feasible. Shifts in wind direction must be taken into account, compounding the magnitude of this approach and making it cost prohibitive.
Because the problem is most acute when material is being dumped onto the pile, other solutions directed specifically to this aspect were also investigated.
The particulate solids are typically dumped onto the pile from an overhead conveyor; and flexible cloth sleeves have been employed to keep the material from becoming airborne as it is dumped. This limited solution proved impractical as the sleeves became imbedded in the particulate material. As a consequence, they could not be withdrawn from the pile and were ripped off as sawdust was extracted from the bottom of the pile.
Rigid chutes were also proposed, but this idea was almost immediately discarded. To be useful, the chute has to be capable of being moved from end-to-end of the pile; also, to prevent air entrainment of fines while material is being dumped on the pile, the chute would have to be capable of being extended where the pile is low and of being retracted where it is high. Such chutes would be difficult to design so that they would not physically interfere with the system provided to dump the particulate material; they would also be costly, if not impractical, to construct in a typical application where variations in the height of the pile might be on the order of fifty feet.